The invention relates to a method of thermal management for an electric battery that is in particular intended for the traction of an electrical or hybrid motor vehicle, that is to say comprising an electric motor driving the drive wheels combined with a thermal engine driving these wheels or possibly other drive wheels.
In particular, the invention applies to a high degree of hybridization of thermal vehicles which may go as far as complete electrification of the traction chain. In this case, the batteries do not then merely serve to assist the vehicles in the acceleration phases but also to provide movement of the vehicle autonomously over greater or lesser distances.
The electric battery can also find its application in other technical fields, for example the storage of electric energy in other modes of transportation, particularly in aeronautics. Moreover, in stationary applications, such as for windmills, the thermal management of a battery according to the invention can also be advantageously used.
To guarantee the levels of power and/or energy required for the applications in question, it is necessary to create batteries comprising a plurality of electric energy generating elements which can be mounted in series.
The generating elements conventionally comprise at least one electrochemical cell, for example of the lithium-ion or lithium-polymer type which can be formed by a stack of electroactive layers acting successively as cathodes and anodes, said layers being put in contact by means of an electrolyte.
However, when these elements are charged and discharged, heat is produced which, when it is not controlled, can decrease the service life of the elements, and, under extreme conditions, can even present risks of thermal runaway for certain chemical compositions of cells, leading to deterioration of the battery.
To optimize the safety, performance, and lifetime of batteries, systems for thermal conditioning the elements have therefore been integrated in the batteries so as to maintain the temperature of said batteries within an optimum temperature range.
Furthermore, in the automotive application that is envisioned, these systems must be very efficient since the thermal dissipation peaks are dependent on the current densities and the variations thereof, which can reach very high values, particularly during phases of strong accelerations, regenerative brakings, rapid recharging of the battery or highway operation in electric mode. In addition, high-energy batteries, which use thick elements whose heat-producing exchange-surface-to-volume-ratio is reduced, must consequently be cooled down in a particularly efficient manner.
In particular, thermal conditioning systems can comprise a chamber formed essentially around the generating elements, in which a fluid for thermal exchange with said elements circulates. In addition, to provide thermal conditioning, the known systems comprise a device for heating and/or a device for cooling the fluid in circulation. This way, by thermally conditioning the fluid and by having a continuous flow of said fluid circulate around the elements, the thermal conditioning of the battery can be carried out.
However, this thermal management strategy leads to the appearance of a temperature gradient within the elements, the amplitude of which is great in high-energy batteries since it depends, among other things, on the:                difference of temperatures between the fluid and the elements;        thickness of the elements;        thermal conduction properties between the core of the elements and the fluid;        thermal power released by the elements in use.        
However, when it becomes too substantial, this temperature gradient causes a thermal imbalance of the elements which leads to a risk for the safety and service life of the battery. Indeed, the local internal capacities and resistances within the elements depend on the local temperature of the latter. The electrochemistry of the elements can thus be stressed in a different manner; a local over-stressing can lead to an acceleration of the aging phenomena.
Furthermore, the thermal conditioning of the battery consumes a substantial part of the electric energy aboard the vehicle. This extra energy consumption causes a loss of autonomy of the electric vehicles. To preserve the autonomy targeted by the application, it can be necessary to compensate this extra consumption by oversizing the battery, which is not cost-effective from a purely economic standpoint.
In addition, high energy Li-ion battery elements have internal resistances which are very sensitive to temperature. Because of this particularity, if the autonomy and performance of the batteries for electric vehicles are to be preserved in cold weather, warming them up by means of the thermal conditioning system becomes necessary. This warming up can also be a source of energy consumption in driving phases.
Finally, increasing the size of the elements to obtain a battery with high energy density can be regarded as placing in parallel stacks of electroactive elements. In case of high inrush currents, the current taking preferably the path with the least resistance, the balancing of the resistances between each elementary branch placed in parallel becomes fundamental.
These differences of internal resistances can lead to a local over-concentration of the current which causes a voltage drop within an element. Since this drop cannot be detected by measuring the overall voltage of the element, this can lead to a risk of exceeding a voltage threshold that is “dangerous” for the electrochemistry of said element.